In order to optimize the accuracy of an analog-to-digital converter (ADC), it is necessary to condition the input analog signal such that its amplitude is just within the full-scale range of the ADC. Input signal amplitude conditioning is conventionally accomplished through use of digitally programmable gain controllers configured in an automatic gain control (AGC) loop. Programmable gain controllers are conventionally based on amplifiers configured with switching resistive feedback networks, known as programmable gain amplifiers (PGA). A drawback associated with such resistive networks is that when monotonic (e.g. no jumps in signal characteristic magnitude) control is needed to condition a particular signal, the corresponding gain control components of the resistive network cannot guarantee monotonicity due to device mismatching when the number of control bits increases to maintain accuracy.
Manufacturing process variations can result in the components (e.g. transistors and resistors) used to fabricate the PGA to have different than expected values or different values relative to corresponding components that are to have the same value; thereby, resulting in corresponding variations in gain amplitude. Variations in gain magnitude can cause the resulting amplifier to exhibit non-monotonic operating characteristics. FIG. 3 is a graph of gain versus a representation of the gain control code for a PGA operating in a non-monotonic region. As illustrated, unwanted gaps (g1, g2) are present in the transfer function of the PGA. These gaps result in the PGA providing unstable output values that, in turn, will result in an erroneous signal being provided to a subsequent ADC. In the case of a video signal that is to be rendered by a graphics processor, an erroneous or otherwise unstable input signal may result, for example, in the resulting image being improperly rendered.
Variable gain amplifiers (VGA) have been used in gain controllers to preprocess analog (i.e. audio or video) signals before conversion by an ADC. Conventional VGAs use charge pumps to control the mapping of voltage into corresponding gain. The advantage of VGA's is that the gain is controlled by a continuous voltage instead of a discrete digital value. This provides an inherently monotonic gain control characteristic. A drawback associated with these conventional VGAs is that they employ a structure including at least two charge pumps that charge a capacitor which, in turn, provides the voltage of the VGA. Capacitors suffer from leakage. Capacitor leakage causes the gain of the VGA to change, sometimes dramatically. This unwanted gain change results in the VGA providing signals of varying magnitude that cannot be effectively controlled or relied upon as being accurate. Additionally, any noise captured by the corresponding charge pumps is passed through the VGA to the signal, further affecting the output of the VGA.
Alternative programmable gain control circuits have been employed to prevent the aforementioned problems associated with conventional VGAs. These gain control circuits control the reference voltage that is applied to the ADC; however, the linearity and signal-to-noise ratio of the ADC output is dramatically reduced when the ADC reference range is reduced. Since the purpose of the AGC is to optimize the analog-to-digital conversion, a gain control scheme that reduces ADC performance is not desirable.
Thus, there is a need for a PGA-based programmable gain control circuit exhibiting operating characteristics that are unaffected by manufacturing process variations and component shortcomings.